ch08, 09, 10: example files

10-dp-1class-func/README.rst

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+Sample code for Chapter 10 - "Design patterns with first class functions"
+
+From the book "Fluent Python" by Luciano Ramalho (O'Reilly, 2015)
+http://shop.oreilly.com/product/0636920032519.do
+
+Notes
+=====
+
+No issues on file with zero type hints
+--------------------------------------
+
+Running Mypy on ``classic_strategy.py`` from the first edition, with no
+type hints::
+
+    $ mypy classic_strategy.py 
+    Success: no issues found in 1 source file
+
+
+Type inference at play
+----------------------
+
+When the ``Order.due`` method made first assignment to discount as ``discount = 0``,
+Mypy complained::
+
+    mypy classic_strategy.py 
+    classic_strategy.py:68: error: Incompatible types in assignment (expression has type "float", variable has type "int")
+    Found 1 error in 1 file (checked 1 source file)
+
+To fix it, I made the first assigment as ``discount = 0``.
+I never explicitly declared a type for ``discount``.
+
+
+Mypy ignores functions with no annotations
+------------------------------------------
+
+Mypy did not raise any issues with this test case::
+
+
+    def test_bulk_item_promo_with_discount(customer_fidelity_0):
+        cart = [LineItem('banana', 30, .5),
+                LineItem('apple', 10, 1.5)]
+        order = Order(customer_fidelity_0, 10, BulkItemPromo())
+        assert order.total() == 30.0
+        assert order.due() == 28.5
+
+
+The second argument to ``Order`` is declared as ``Sequence[LineItem]``.
+Mypy only checks the body of a function the signature as at least one annotation,
+like this::
+
+    def test_bulk_item_promo_with_discount(customer_fidelity_0) -> None:
+        cart = [LineItem('banana', 30, .5),
+                LineItem('apple', 10, 1.5)]
+        order = Order(customer_fidelity_0, 10, BulkItemPromo())
+        assert order.total() == 30.0
+        assert order.due() == 28.5
+
+
+Now Mypy complains that "Argument 2 of Order has incompatible type".
+
+However, even with the annotation in the test function signature,
+Mypy did not find any problem when I mistyped the name of the ``cart`` argument.
+Here, ``cart_plain`` should be ``cart``::
+
+
+    def test_bulk_item_promo_with_discount(customer_fidelity_0) -> None:
+        cart = [LineItem('banana', 30, .5),
+                LineItem('apple', 10, 1.5)]
+        order = Order(customer_fidelity_0, cart_plain, BulkItemPromo())
+        assert order.total() == 30.0
+        assert order.due() == 28.5
+
+
+Hypotesis: ``cart_plain`` is a function decorated with ``@pytest.fixture``,
+and at the top of the test file I told Mypy to ignore the Pytest import::
+
+    import pytest  # type: ignore